Field of the Invention
The present invention relates to an electrical machine having a stator and having a rotor with an axis of rotation. An electrical machine can be, for example, a brushless electric motor (DC motor) or a synchronous machine, but a generator as well.
Description of the Background Art
In the automotive industry or in motor vehicles, electric motors (DC motors) are used in drives for various actuating elements, for example, as window lift, sunroof, or seat adjustment drives and/or as a steering wheel drive (electric power steering), as a cooling fan drive, or as a transmission actuator. Such electric motors have a relatively high torque or power density and should be reliable to operate particularly also at relatively high engine temperatures of, for example, up to at least 120° C.
In principle, brushed electric motors (commutator motors) and/or brushless electric motors, whose rotor, surrounded by a stator provided with a field winding, is equipped or provided with permanent magnets, are used in the automotive sector. Typically, both the rotor and stator are made as lamination stacks, whereby stator teeth with stator slots lying inbetween support the coils of the field winding. These are controlled by an (electronic) circuit in order to generate a rotating field which produces a torque on the permanently excited rotor.
Because both high operating temperatures and (external) magnetic fields, which are caused, for example, by current peaks induced in the stator-side field winding or coil, can lead to an undesirable end magnetization of the rotor-side permanent magnet, magnet materials or alloys with as high a coercive field strength as possible are usually used. To achieve as high a coercive field strength as possible, therefore, as a rule alloys with components or amounts of rare earth elements are used. In this regard, both light rare earth elements (LREE), particularly samarium-cobalt or neodymium-iron-boron alloys, as well as terbium- or dysprosium-containing alloys are employed to increase the coercivity and to broaden the usable temperature range. In the meantime, however, the so-called heavy rare earth elements (HREE) such as, for example, terbium and particularly dysprosium, have become increasingly cost-intensive.